1. Field of the Invention
This invention relates to isotope separation. More particularly, the invention relates to isotope separation using a laser or laser system to initially pump a selected isotope to an excited state of long life time (metastable state) and then supplying a continuous source of reduced ionization energy to form a plasma of the isotope. The selected isotope plasma may then be separated from the isotope in the neutral state by applying external electromagnetic fields.
2. Description of Prior Art
Uranium enriched with the isotope .sup.235 U is increasingly needed as fuel for nuclear reactors. Two enrichment methods are predominantly used. The slight mass differences of .sup.235 U from .sup.238 U are used by the gaseous diffusion and centrifuge methods. Enrichment by these methods generally require cascaded processing using a sequence of many stages in series, each stage providing only a slight increase in the concentration of the desired isotope due to the small separation factor per separating unit at each stage.
Another method is one of photo-ionization in which the atoms of the desired isotope are excited by one source of radiant energy and the excited isotope atoms are then ionized by a second source of radiant energy. The two radiant energy sources are generally tunable lasers. The desired ions are then removed by application of electromagnetic fields. The drawbacks of this method are the small cross-section for photo-ionization and the extremely short lifetimes of the isotopes in the excited state. The short lifetime requires application of powerful pulses of exciting and ionizing radiation at different wavelengths.
A promising new technique for efficient production of enriched uranium isotopes operates by first exposing a vapor of uranium to a laser radiation source to excite a specific isotope to an excited state above the ground state. In cascading down to lower energy states many electrons will be trapped in long-life metastable states. By intense pumping with laser radiation a large fraction of the selected isotope will accumulate in a metastable state. Several alternative embodiments are described in which a continuous or pulsed source of `reduced ionization energy` is imparted to the specific isotope in the excited and metastable state to create an ion plasma of the specific isotope. The nature of the ionization source avoids the problems listed above and makes maximal use of those isotopes in the metastable excited state having much longer lifetimes. Electromagnetic forces are then applied to separate the ion plasma from the neutral vapor. The `reduced ionization energy` supplied is described by the equation: EQU .xi..sub.i *.gtoreq..xi..sub.i -.xi.*
where
.xi..sub.i * is the reduced ionization energy PA0 .xi..sub.i is the ionization energy necessary to ionize the atom from the ground state. PA0 .xi.* is the energy of the isotope in the excited state which depends on the laser photon energy h.nu.. PA0 .xi.*&lt;h.nu. when the electron of the excited isotope is trapped in a metastable state below the pump level.